The ubiquitous coverage that wireless communication can be held anywhere in the world, which is an ability of communication terminal equipment, such as mobile phone terminals, has been not a reality today, but under development.
Mobile systems for actualizing the ubiquitous are cellular phones for e.g. GSM (Global System for Mobile Communication), PCS (Personal Communication System), DCS (Digital Cellular System), GPRS (General Packet Radio Service), EDGE (Enhanced Data for GSM Evolution; Enhanced Data for GPRS) and WCDMA (Wideband Code Division Multiple Access). These systems cover, in properties, signals with fixed and changing envelops, multiplexes of time division and code division and a wide range of combinations of transmit-output powers ranging HIGH (several watts) to LOW (microwatts). Consequently, the demands for multiband and multimode applications have been growing.
Meanwhile, Non Patent Citation 1, which is cited later, describes an antenna switch microwave monolithic IC (MMIC) for GSM, DCS, PCS and WCDMA quad-bands. MMIC can handle transmit and receive signals of GSM, DCS and PCS systems in series according to the means of time division, and it can treat transmit and receive signals of WCDMA system in parallel by code division. HEMT (High Electron Mobility Transistor) of a heterojunction structure having a low on-resistance is used for the switch, in which AlGaAs is used as a barrier layer, and InGaAs is used as a channel layer. Also, it is described in Non Patent Citation 1 that second and third harmonic distortions at DCS and PCS2 transmit terminals are about −70 dBc.
Further, Patent Citation 1, which is cited later, describes a switch circuit for a transmitter-receiver switching circuit for a radio communication system, in which a high voltage from a switch circuit in ON state is prevented from first turning on a high-voltage-side FET of serially connected FETs of a switch circuit in OFF state by making gate resistances of the serially connected FETs smaller from the high-voltage side toward the low-voltage side in turn. Thus, it becomes possible to provide an electronic part for communication smaller in insertion loss and harmonic distortion.
Further, Patent Citation 2, which is cited later, describes an RF switch circuit for mobile communication devices, in which of serially connected FETs of a high-frequency switch circuit in OFF state, only the FET near to the input/output terminal and accepting application of a high voltage from the high-frequency switch circuit in ON state has its gate resistance set to the maximum, and the gate resistances of the others are set below the maximum. Thus, even in case that the total of resistance values of the gate resistances is made smaller, the influence on a signal path can be reduced.
In addition, Patent Citation 3, which is cited later, describes a switch circuit for mobile communication devices. The switch circuit includes two or more FETs each having a plurality of gates, in which a drain's additional capacitance is connected between the drain of FET and the gate adjacent to the drain, and a source's additional capacitance is connected between the source of FET and the gate adjacent to the source. The drain's additional capacitance between the drain of FET of a switch in OFF state and the gate adjacent to the drain can suppress the phenomenon that the negative direction's voltage fluctuation from the switch circuit in ON state causes FET of a switch in OFF state to be turned on. In addition, the source's additional capacitance between the source of FET of a switch in OFF state and the gate adjacent to the source can suppress the phenomenon that the positive direction's voltage fluctuation from the switch circuit in ON state causes FET of a switch in OFF state to be turned on. Thus, a high-frequency switch with low voltage and low distortion characteristics can be materialized. Now, it is noted that Patent Citation 3 corresponds to U.S. Pat. No. 5,774,792.
Further, Patent Citation 4, which is cited later, describes an antenna switch circuit for wireless communication devices including portable terminals. FET of the antenna switch circuit includes a multi-gate transistor having a plurality of gates between its drain and gate. An inter-gate region located between adjacent gates of the plurality of gates is connected to the drain and source through a potential-stabilizing resistance, whereby the signal leakage between the drain and source of a multi-gate type FET can be suppressed.
The ubiquitous coverage that wireless communication can be held anywhere in the world, which is an ability of communication terminal equipment, such as mobile phone terminals, has been not a reality today, but under development.
There are a variety of systems as mobile systems for actualizing the ubiquitous, which include cellular phones for e.g. GSM (Global System for Mobile Communication), PCS (Personal Communication System), DCS (Digital Cellular System), GPRS (General Packet Radio Service), EDGE (Enhanced Data for GSM Evolution; Enhanced Data for GPRS) and WCDMA (Wideband Code Division Multiple Access), and systems for e.g. wireless LAN (Local Area Network) and WIMAX (Worldwide Interoperability for Microwave Access).
These systems cover, in properties, signals with fixed and changing envelops, multiplexes of time division, frequency division, code division and the like and a wide range of combinations of transmit-output powers ranging HIGH (several watts) to LOW (microwatts). Consequently, the demand to make terminals for such systems multimode-ready ones which are each capable of supporting multiple systems has been growing. The needs for multiband and multimode applications have been growing. As to mobile communication terminals supporting a system which adopts a time-division transmit-and-receive technique that transmission and reception are switched by means of time division, and mobile communication terminals which perform communication supporting multimodes, it is necessary to switch between transmit and receive modes. For such switching, an antenna switch is used.
Mobile terminals are driven by batteries, and therefore they are required to lower power consumption. In a mobile communication terminal, what consumes electric power most is a power amplifier operable to amplify the power of a transmit signal to a power as large as several watts. For reduction in power consumption, it is effective to raise the power conversion efficiency of such power amplifier. However, it is also effective to supply an amplified signal to an antenna through an antenna switch with a smaller loss and then throw out it into space in terms of the improvements of the power conversion efficiency and greater power savings. Hence, an antenna switch connected between a power amplifier and an antenna is required to have a smaller loss.
Radio wave resources are managed and operated by each country or area. As to mobile communication terminals which emit radio waves into space, the frequency of radio waves and the strength of power which they can use for respective systems are specified by each country or district. Therefore, the strength of power emitted into space at frequencies except a frequency used for a system of e.g. harmonic power needs to be controlled to or below a value stipulated by a law or the like. The power emitted by such terminal is amplified by a power amplifier, passed through an antenna switch, and radiated from an antenna. Usually, harmonics generated in a power amplifier can be reduced by LPF (Low Pass Filter) in an output part of the power amplifier sufficiently. However, the harmonic distortion caused by an antenna switch connected with an output of LPF is emitted into space through the antenna as it is. Hence, an antenna switch needs to suppress the occurrence of harmonic distortion, i.e. to have the performance of high linearity.
While antenna switches using PIN diodes have been common conventionally, GaAs switch FETs (Field Effect Transistors), which are higher in processing speed than PIN diodes, have been used for microwave signal switches as described in Non Patent Citation 2, which is cited later.
However, GaAs switch FETs have the problem that the breakdown voltage is much lower than that of PIN diodes. Hence, Non Patent Citation 2 describes a technique to resolve the problem by connecting, in series, lots of FET cells and making smaller a voltage applied to each row of FETs in a GaAs microwave monolithic IC (MMIC).
Further, in a GaAs monolithic switch IC, a waveform distortion occurs as transmit power increases. Hence, Non Patent Citation 3, which is cited later, describes a switch including a feedforward circuit to solve the problem of waveform distortion. In the switch, a drain-source path of a first FET is connected between an RF signal input terminal and a ground voltage, and a source-drain path of a second FET is connected between the RF signal input terminal and an RF signal output terminal. The feedforward circuit includes a feedforward capacitance and a diode, which are connected in series, between the RF signal input terminal and the gate of the first FET. In case that an RF signal is not transmitted from the RF signal input terminal to the RF signal output terminal, the first FET is controlled to be in ON, and the second FET is controlled to be in OFF. In contrast, in case that an RF signal is transmitted from the RF signal input terminal to the RF signal output terminal, the first FET is controlled to be in OFF, and the second FET is controlled to be in ON. During the RF signal transmission, the low level of an RF signal at the RF signal input terminal is sent to the gate of the first FET through the feedforward circuit as a negative voltage. Thus, the problems of waveform distortion and RF transmit power loss can be avoided.
It is described in Patent Citation 5, which is cited later, to connect DC boost circuits to an RF switch including switch elements connected with RF signal sources. The switch elements are composed of FETs, and a DC control voltage is applied to the gate of each FET for ON-OFF control. In general, the DC control voltage is produced from a voltage of a system power source. In case that the DC control voltage lowers to or below 2.5 volts, harmonic signal components, which cause distortion in an RF output signal, increase remarkably. In Patent Citation 5, the DC boost circuit including diodes, capacitances and resistances is supplied with a DC control voltage and an RF signal. A DC output voltage larger than the DC control voltage is extracted from the DC boost circuit according to charge and discharge operations by the diodes and capacitances in response to positive and negative voltages of an RF signal. The resistances make the input impedance of the DC boost circuit high to prevent a large current from flowing from the RF signal source into the DC boost circuit.
[Non Patent Citation 1] Hiroyuki Tosaka et al., “An Antenna Switch MMIC Using E/D Mode p-HEMT for GSM/DCS/PCS/WCDMA Bands Application”, 2003 IEEE Radio Frequency Integrated Circuits Symposium, PP. 519-522.
[Non Patent Citation 2] M. B. Shifrin at al., “Monolithic FET Structures for High-Power Control Component Applications”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 37, NO 12, DECEMBER 1989, PP. 2134-2141.
[Non Patent Citation 3] K. Miyatsuji at al., “A GaAs High-Power RF Single-Pole Double-Throw Switch IC for Digital Mobile Communication System”, 1994 IEEE International Solid-State Circuit Conference DIGEST OF TECHNICAL PAPERS, PP. 34-35.
[Patent Citation 1] JP-A-2005-072671
[Patent Citation 2] JP-A-2006-174425
[Patent Citation 3] JP-A-8-70245
[Patent Citation 4] JP-A-2000-101032
[Patent Citation 5] US Patent Application No. US2004/0229577A1